Heretofore, there has been known a friction clutch that performs a power transmission by frictionally engaging clutch plates on a driving side with clutch plates on a driven side. Further, there has been also known a friction clutch of an electromagnetic type that uses a clutch plate formed with a plurality of windows being arc-shaped through holes. The windows are required to complete a magnetic circuit. The friction clutch of the electromagnetic type is described in JP11-303911A for example.
Further, an electronically controlled 4WD coupling (ITCC (registered trademark)) using an electromagnetic friction clutch is also described in JP2002-213485A for example.
Lubrication oil is interposed between the clutch plates. Then, lubrication grooves are formed on end surfaces (sliding surface) in the axial direction of the clutch plates for retaining the lubrication oil and for discharging the same from between the plates at the time of friction engagement of the plates with each other.